1. Field
This disclosure relates generally to non-volatile memories (NVMs), and more specifically, to programming multi-state NVMs.
2. Related Art
Multi-state NVMs, in which one memory cell represents more than one bit, provide for efficient use of space and thus are cheaper for a given amount of memory. A multi-state memory in the case of an NVM will have one erased state and multiple programmed states. If there are two programmed states, then there are a total of 3 possible states, two programmed and one erased. If there are three possible programmed states, then there are a total of 4 possible states so that each cell represents 2 bits. One of the issues with multiple programmed states is the amount of time required to achieve the desired programmed states. With sufficient separation in threshold voltage (Vt), programming is a relatively lengthy process even for single-bit cells, but multiple programmed states significantly further complicates the programming process. There must be enough margin to ensure that the various states have threshold voltage distributions sufficiently separated from each other so that they can be read reliably and quickly. So the programming process must result in programmed cells that can be efficiently read.
Accordingly, it is desirable to provide a multi-state NVM that improves upon one or more of issues raised above.